For accuracy in the OCLT make sure you fill the 10ml exactly the same in each test. If you are 1/4ml high on the first and 1/4 ml low on the second you'll be off by 1.5 FC error right out of the gate at fc=30. 1/4 ml on those lines is not much!
Hold the bottle vertical when doing the drops. And make sure they hang for a bit before dropping. Otherwise the drop sizes can be a further source of errors affecting the repeatability of the test.
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